1. Technical Field
The invention relates to single crystal thin film ternary nitride semiconductors and devices made from them.
2. Background
In addition to the semiconductors, C, Ge, Si and .alpha.Sn selected from Group IV of the Periodic Table, a number of useful ones have been produced using binary, ternary, and multinary compounds of other Group elements. The Handbook of Chemistry and Physics, 77th ed, pp 12:90-100, (1996-1997), David R. Lide, ed., CRC Press, New York, London, Tokyo lists various properties of about 200 semiconductors not including alloys. Most of these are not yet as useful as silicon for making electronic devices but have found use in making various optical and electro-optical devices.
The Group III-V compound, GaN, is in important example of a binary semiconductor and in many respects related to the subject of this invention. Thus, a discussion of its growth and use may be instructive background. Unlike with silicon which can be grown as bulk crystals and cut into wafers for further processing, it is often desirable to produce devices from other semiconductors as thin (less than about 20 .mu.m) films. The metalorganic chemical vapor deposition (MOCVD) method has been used to grow a wide variety of thin film compounds and, since about 1970, has been used to grow GaN. For a slightly out of date but thorough discussion, see Organometallic Vapor Phase Epitaxy, (1989), Gerald B. Stringfellow, Academic press, New York.
In the MOCVD method, metal organic precursors, e.g., triethylgallium and ammonia (NH3) are injected into a reaction chamber containing a heated substrate, e.g., single crystal sapphire (Al.sub.2 O.sub.3). It has been found that by using suitable substrates, growth of single crystal films will be promoted. Moreover, additional films of different composition can be grown on an underlying film. This method works best when the size of the two crystal lattices are matched. Because the sapphire lattice is 49% bigger than the GaN lattice, growth of high quality single crystal GaN on sapphire is difficult. Unfortunately, for reasons of cost, ruggedness, high melting point and, sometimes, transparency and electrical insulation, sapphire is a highly desirable substrate. Thus methods of growing GaN have resorted to first growing a thin (ca. 20 nm) buffer layer on which additional films are grown. However, such an approach is usually not possible when growing an active layer on another active layer, because the buffer layer could interfere with the junction.
Because the nitride semiconductors have wider energy gaps than silicon or the III-V arsenides and phosphides, UV and blue LEDs and injection lasers are possible. In order to produce blue light emission, GaN must be alloyed with some In to reduce the energy gap. High brightness LEDs have been made using heterostructures comprising layers of GaN and the alloys, Al.sub.X Ga.sub.1-X N and In.sub.Y Ga.sub.1-Y N, Nakamura et al, Appl. Phys. Lett., vol 64, no. 13, pp 1687-1689, (1994).
The wider energy gaps also mean that electronic amplifier and switching devices based on nitrides can remain operational at much higher temperatures. Furthermore, these nitride materials have higher thermal conductivities than Si, GaAs, or InP, and therefore can handle greater power levels. The nitride semiconductors can also withstand much higher levels of reverse bias voltages without breaking down. It has been shown that the III-V nitrides possess many of the most desirable properties for application to high temperature electronic devices K. Shenai et al, IEEE Trans. Electron. Dev., vol. 36, pp. 1811-1823 (1989). Simple GaN field-effect transistors using Schottky barrier gate contacts have been reported, M. Asif Khan et al, Appl. Phys. Lett. vol. 62, pp. 1786-1788 ( 1993). However, the maximum achievable gains and bandwidths will remain limited unless heterojunction transistors can be manifested. Much superior operating characteristics are generally expected from transistors featuring heterojunctions between a wider energy gap charge supply layer and a narrower energy gap undoped charge transit channel, a so-called "high electron mobility transistor" or HEMT (also called a modulation-doped-field-effect-transistor or MODFET). Such a HEMT using a GaN channel and an AlN--GaN alloy film as the charge supply film has now been demonstrated, M. Asif Khan et al, Appl. Phys. Lett., vol. 63, pp. 1214-1216 (1993). Furthermore, the gain of a bipolar transistor is improved if the emitter-base heterojunction is chosen so as to impede back injection of charges from the base into the emitter, a so-called "heterojunction bipolar transistor," or HBT.
The In.sub.x Ga.sub.1-x N alloys provide a wide range of choices of energy gaps, ranging from 1.9 eV (InN) to 3.4 eV (GaN). Unfortunately, in close analogy with the GaAs/InAs alloy series, InN has a significantly larger lattice parameter than GaN (a=3.548 .ANG. vs. a=3.189 .ANG., respectively), and therefore such alloys grown on GaN must suffer from increased numbers of structural (and hence potentially electrically active) defects due to the mismatch. Such defects may be expected to impede high frequency and high power device applications. Another problem with GaN is that it is hard to make heavily doped p-type layers so that forming electrical contacts is difficult.
Given the problems with GaN based devices, it would be useful to have another compound which has a better lattice match to GaN and may possible have other useful properties. Preferably it should be a nitride for ease of MOCVD processing and because it would more likely grow epitaxially on GaN. While the binary III-V nitride compounds have been extensively investigated, no ternary compounds have. One could be formed by substituting elements on either side of Ga in the Periodic Table, namely Zn (Group II) and Ge (Group IV) to form ZnGeN.sub.2. Another possibility is ZnSiN.sub.2. In addition to their use with GaN based devices, these ternary nitrides should have uses on their own. Single crystal thin films of these compounds were not heretofore known but the phosphide, ZnGeP.sub.2 is.
ZnGeP.sub.2 is used extensively for frequency shifting applications due to its high birefringence and substantial nonlinear optical figure of merit, G. D. Boyd et al, Appl. Phys. Lett., vol. 18, pp. 301-303 (1971). MOCVD growth of ZnGeP.sub.2 has been reported, G. C. Xing et al, Mat. Res. Soc. Symp. Proc., vol. 162, pp. 615-620 (1990). All MOCVD samples proved to be low resistivity p-type, with carrier concentrations around 1.times.10.sup.18 cm.sup.-3 ; whereas, samples prepared by sublimation and transport in a closed tube proved to be n-type. Thus, changes in the growth conditions can shift the conductivity type for ZnGeP.sub.2 from p- to n-type; and clearly, the Zn/Ge ratio will be critical. A deficiency of Zn leads to vacancies on the cation lattice and hence the formation of acceptors (p-type). A stoichiometric Zn/Ge ratio with P vacancies will give donors (n-type). The extra degree of freedom presented by the ternary II-IV-V.sub.2 compound can allow n- and p-type conductivity to be manifested without the need for extrinsic dopants. Although the result is not certain, by analogy, ZnGeN.sub.2 and the other II-IV-N.sub.2 materials may also provide both types of doping with control afforded through manipulation of the, for example, Zn/Ge ratio in the vapor.
An early report of ZnGeN.sub.2 powders was given by M. Maunaye et al, Mat. Res. Bull., vol. 5, pp. 793-796 (1970), who prepared ZnGeN.sub.2 by reacting zinc metal with Ge.sub.3 N.sub.4 at 750.degree. C. Their product was contaminated with excess Ge. This same group later reported that the structure was wurtzite (as is GaN) with lattice constants of a=b=3.167 and c=5.194 which are close to GaN, M. Wintenburger et al, Mat. Res. Bull., vol. 8, pp. 1049-1054 (1973).
Only one group has reported the preparation of ZnGeN.sub.2 in thin film form. They reacted elemental zinc and germanium with gaseous hydrogen chloride (HCl) and allowed these zinc and germanium chlorides to react with ammonia gas in a tube furnace,. W. L. Larson et al, J. Electrochem. Soc., vol.121, pp. 1673-4 (1974). They used sapphire as a substrate, and produced poly-crystalline thin films with a pale yellow color. The energy gap was suggested to be 2.67 eV, based on optical transmission measurements.
Until recently, the existence of ZnSiN.sub.2 was not known. However, T. Endo et al, J Mat. Sci. Lett., vol. 11, pp. 424-426 (1992), mixed Zn.sub.3 N.sub.2 with either Si.sub.3 N.sub.4 or Ge.sub.3 N.sub.4 powders and heated them at 1000-1600.degree. C. under an extremely high pressure of 4-6.5 GPa for several hours and report recovering both ZnGeN.sub.2 and ZnSiN.sub.2. Their samples of ZnGeN.sub.2 were black and conducting, probably due to a lack of stoichiometry (nitrogen loss during the reaction). They also reported quaternary alloys of the form ZnSi.sub.(1-X) Ge.sub.X N.sub.2, T. Endo et al, Ceram. Eng. Sci. Proc., vol. 13, pp. 844-851 (1992).